***Useful Information for Apprentices***

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“ General Health and Safety at Work “

Question 1.1
What do the letters CDM stand for ?
A: Control of Demolition and Management Regulations
B: Control of Dangerous Materials Regulations
C: Construction (Demolition Management) Regulations
D: Construction (Design and Management Regulations ) Answer: D )
Question 1.2
Identify one method of enforcing regulations that are
available to the Health and Safety Executive:
A: Health Notice
B: Improvement Notice
C: Obstruction Notice
D: Increasing insurance premiums
Answer: B Improvement notices require action to achieve standards which meet health and safety law :
Question 1.3
What happens if a Prohibition Notice is issued by an
Inspector of the local authority or the HSE ?
A: The work in hand can be completed, but no new work started
B: The work can continue if adequate safety precautions are put in place
C: The work that is subject to the notice must cease
D: The work can continue, provided a risk assessment is carried out,
Answer: C The work covered by a prohibition notice must cease until the identified danger is removed.
Question 1.4
Health and Safety Executive Inspector can ?
A: Only visit if they have made an appointment
B: Visit at any time
C: Only visit if accompanied by the principal contractor
D: Only visit to interview the site manager
Answer: B Inspectors have a range of powers, including the right to visit premises at any time.
Question 1.5
A Prohibition Notice means:
A: When you finish the work you must not start again
B: The work must stop immediately
C: Work is to stop for that day only
D: Work may continue until the end of the day
Answer: B The work activity covered by the prohibition notice must cease, until the identified danger is removed ,
Question 1.6
In what circumstances can an HSE Improvement Notice be issued ?
A: If there is a breach of legal requirements
B: By warrant through the police
C: Only between Monday and Friday on site
Answer: A Improvement notices require action to achieve standards which meet health and safety law .
Question 1.7
What is an “Improvement Notice”?
A: A notice issued by the site principal contractor to tidy up the site
B: A notice from the client to the principal contractor to speed up the work
C: A notice issued by a Building Control Officer to deepen foundations
D: A notice issued by an HSE/local authority Inspector to enforce compliance with health
Answer: D Improvement notices require action to achieve standards which meet health and safety law .
Question 1.8
If a Health and Safety Executive Inspector issues a“ Prohibition Notice”, this means that:
A: the Site Manager can choose whether or not to ignore the notice
B: specific work activities, highlighted on the notice, must stop
C: the HSE must supervise the work covered by the notice
D: the HSE must supervise all work from then on
Answer: B Prohibition notices are intended to Stop activities which can cause serious injury.
Question 1.9
Which one of the following items of information will you find on the Approved Health and Safety Law poster?
A: Details of emergency escape routes
B: The location of the local HSE office
C: The location of all fire extinguishers
D: The identity of the first aiders
Answer: B The poster also lists the persons with health and safety responsibilities, but not first aiders.
Question 1.10
Who is responsible for signing a Company Safety Policy ?
A: Site Manager
B: Company Safety Officer
C: Company Secretary
D: Managing Director
Answer: D The Health and Safety at Work Act requires the most senior member of management to sign the health and safety policy
statement.

Question 1.11
Which one of the following must be in a company’s written Health and Safety Policy:
A: Aims and objectives of the company
B: Organisation and arrangements in force for carrying out the health and safety policy
C: Name of the Health and Safety Adviser
D: Company Director’s home address
Answer: B This requirement appears in the Health and Safety at Work Act.
Question 1.12
Employers have to produce a written Health and Safety Policy statement when:
A: A contract commences
B: They employ five people or more
C: The safety representative requests it
D: The HSE notifies them
Answer: B This is a specific requirement of the Health and Safety at Work Act.
Question 1.13
Companies employing five or more people must have a written Health and Safety Policy because:
A: The principal contractor gives them work on site
B: The HSAWA 1974 requires it
C: The Social Security Act requires it
D: The trade unions require it
Answer: B
Question 1.14
What do the letters HSC stand for ?
A: Health and Safety Contract
B: Health and Safety Consultant
C: Health and Safety Conditions
D: Health and Safety Commission Answer: D
Question 1.15
Which ONE of the following statements is correct ? The Health and Safety Executive is:
A: a prosecuting authority
B: an enforcing authority
C: a statutory provisions authority
Answer: B The Health and Safety Executive enforces health and safety legislation.
Question 1.16
The Health and Safety at Work Act requires employers to provide what for their employees?
A: Adequate rest periods
B: Payment for work done
C: A safe place of work
D: Suitable transport to work
Answer: C This is a specific requirement of Section 2 of the Health and Safety at Work Act.
Question 1.17
The Health and Safety at Work Act 1974 and any regulations made under the Act are:
A: Not compulsory, but should be complied with if convenient
B: Advisory to companies and individuals
C: Practical advice for the employer to follow
D: Legally binding Answer: D
Question 1.18
Under the Health and Safety at Work Act 1974, which of the following have a duty to work safely?
A: Employees only
B: The general public
C: Employers only
D: All people at work
Answer: D Employers, employees and the self-employed all have a duty to work safely under the Act.
Question 1.19
What is the MAXIMUM penalty that a Higher Court, can currently impose for a breach of the Health and Safety at Work Act?
A: £20,000 fine and two years imprisonment
B: £15,000 fine and three years imprisonment
C: £1,000 fine and six months imprisonment
D: Unlimited fine and two years imprisonment
Answer: D A Lower Court can impose a fine of up to £20,000 and/or up to six months imprisonment for certain offences. The potential fine in a Higher Court, however, is unlimited and the term of imprisonment can be up to 2 years.
Question 1.20
What do the letters ACoP stand for ?
A: Accepted Code of Provisions
B: Approved Condition of Practice
C: Approved Code of Practice
D: Accepted Code of Practice
Answer: C An ACOP is a code of practice approved by the Health and Safety Commission.

Question 1.21
Where should you look for Official advice on health and safety matters?
A: A set of health and safety guidelines provided by suppliers
B: The health and safety rules as laid down by the employer
C: Guidance issued by the Health and Safety Executive
D: A professionally approved guide book on regulations
Answer: C The HSE is the UK enforcing body and its guidance can be regarded as ‘official’
Question 1.22
Regulations that govern health and safety on construction sites:
A: apply only to inexperienced workers
B: do not apply during ’out of hours’ working
C: apply only to large companies
D: are mandatory ( that is, compulsory )
Answer: D The requirements of health and safety law are mandatory, and failure to follow them can lead to prosecutions.
Question 1.23
Which of the following statements is correct ?
A: The duty for health and safety falls only on the employer
B: All employees must take reasonable care, not only to protect themselves but also their colleagues
C: Employees have no responsibility for Health and Safety on site
D: Only the client is responsible for safety on site
Answer: B The responsibility for management of Health and Safety Act at Work rests with the employer
Question 1.25
Which of the following is correct for risk assessment?
A: It is a good idea but not essential
B: Only required to be done for hazardous work
C: Must always be done
D: Only required on major jobs
Answer: C There is a legal requirement for all work to be suitably risk assessed.
Question 1.26
In the context of a risk assessment, what do you understand by the term risk?
A: An unsafe act or condition
B: Something with the potential to cause injury
C: Any work activity that can be described as dangerous
D: The likelihood that harm from a particular hazard will occur
Answer: D Hazard and risk are not the same. Risk reflects the chance of being harmed by a hazard
Question 1.27
Who would you expect to carry out a risk assessment on your working site?
A: The site planning supervisor
B: A visiting HSE Inspector
C: The construction project designer
D: A competent person
Answer: D A risk assessment must be conducted by a 'competent person’.
Question 1.28
What is a HAZARD ?
A: Where an accident is likely to happen
B: An accident waiting to happen
C: Something with the potential to cause harm
D: The likelihood of something going wrong
Answer: C Examples of hazards include: a drum of acid, breeze blocks on an elevated plank; cables running across a floor.
Question 1.29
What must be done before any work begins ?
A: Emergency plan
B: Assessment of risk
C: Soil assessment
D: Geological survey
Answer: B This is a legal requirement of the Management of Health and Safety at Work Regulations.
Question 1.30
Complete the following sentence: A risk assessment
A: is a piece of paper required by law
B: prevents accidents
C: is a means of analysing what might go wrong
D: isn’t particularly useful
Answer: C Risk assessment involves a careful review of what can cause harm and the practical measures to be taken to reduce the risk of harm.

 

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Electrical Appliances

Earthed Equipment
In Practice, the Most Common Instances of Faulty Earthing are:-
1) Earth connections broken accidentally or corroded through age.
2) Earth connections incorrectly made.
3) Earth connections not made at all.
4) Earth connections removed for some specific purpose and not reinstated.

 

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2330 – level 3 Unit 1 . 301 / paper 3 :- some old papers 8 Flemings left hand rule can be used to determine the :-
a) emf generated in a conductor within a magnetic field.
b) movement of a conductor within a magnetic field.
c) current flowing in a conductor within a magnetic field.
d) angle of the conductor within a magnetic field.
9 Shunt wound d.c. motors and generators have shunt field coils :-
a) connected across the supply terminals.
b) connected in parallel with the armature.
c) connected in series with the armature.
d) connected in parallel with the rotor.
10 The résistance of a 2.5mm2 copper conductor of length 8m and resistivity p = 1.72 x 10-8 Ωm is :-
a) 0.055 Ω. B) 0.55 Ω. C) 0.12 Ω. D) 5.56 Ω.
11 A circuit consisting of an inductance of 0.35H connected to a 50Hz has an inductive reactance of :-
A) 9.1 mΩ. b) 0.09 Ω. C) 109.9 Ω. D) 1.099 kΩ.
12 When carrying out volt drop calculations the symbol ( Iz ) represents the :-
a) design current.
b) current rating of the protective device.
c) overload current for the circuit.
D) current carrying capacity of the cable.
13 the Star rotor windings of a three-phase wound rotor induction motor are connected to a bank of external resistors by :-
a) slip rings.
b) a commutator and slip rings.
c) slip rings and brushes.
d) an armature and brushes.
14 A series a.c. circuit of 12Ω résistance , 6Ω inductive reactance and capacitive reactance of 3Ω . the impedance is :-
a) 13 Ω. B) 3.6 Ω. C) 153 Ω. D) 12.37 Ω.
15 An eight pole-motor operating on a 50Hz supply would have a synchronous speed of :-
a) 750 rpm. b) 3000 rpm. c) 1500 rpm. d) 2400 rpm.
16 The junction voltages for silicon diodes is approximately :-
a) 0.6V. b) 0.3V. c) 0.7V. d) 0.2V.
17 A short circuit occurring in the primary winding of an auto-transformer could cause :-
a) the supply voltage to be developed across the secondary.
b) a reduced secondary voltage.
c) a reduction of the secondary current.
d) oscillations in the output voltage.
18 The formula for calculating power dissipated in a resistor is :-
A_ P = I/R. b) P = V²R. c) P = l²R. d) P = IR².
19 Fires in the workplace may be caused by :-
a) waste disposal systems.
b) an accumulation of rubbish.
c) storing materials and packaging in designated areas.
d) preventive maintenance practices.
20 Semiconductor devices are sometimes described as :-
a) accessories. B) electronic components. C) resistors. D) devices.
21 MCBs operate on which of the following principles :-
a) thermal and magnetic.
b) thermal.
c) either thermal or magnetic.
d) magnetic.
22 Whilst working in confined spaces you :-
a) should bring a fire extinguisher.
b) use low voltage equipment.
c) use amplification equipment.
d) should be accompanied.
23 Transformers are rated in :-
kWh. B) kVA. C) kW. D) kVAr.
24 Supplies connected to the stator windings in three-phase induction motors create :-
a) opposing magnetic fields.
b) stationary magnetic fields.
c) in phase magnetic fields.
d) rotating magnetic fields.

 

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The Electricity at Work Regulations 1989 came into force on 1st April 1990; their purpose is to require precautions to be taken against the risk of death or personal injury from electricity, in work activities.

In the main, the Regulations are concerned with the prevention of danger from electric shock, electric burn, electrical explosion or arcing or from fire or explosion initiated by electric energy.

All places of work covered by the Health and Safety at Work Act (shops, offices, factories, workshops, farms, garages, sports and entertainment centres, etc.) are covered under these Electricity at Work Regulations.

Electricity at Work Regulations 1989
Safe Working Practices

WHAT THE REGULATIONS COVER

Some of the regulations are absolute (see * below), which means they must be complied with regardless of costs or other considerations. If you utilise electricity in the workplace you will be affected.

Systems, work activities and protective equipment (Reg 4) (4.4)*
Strength and capability of electrical equipment (Reg 5)*
Adverse or hazardous environments (Reg 6)
Insulation, protection and placing of conductors (Reg 7)
Earthing or other suitable precautions (Reg 8)
Integrity of referenced conductors (Reg 9)
Connections (Reg 10)*
Means for protecting from excess current (Reg 11)*
Means of cutting off the supply and for isolation (Reg 12)*
Precautions for work on equipment made dead (Reg 13)*
Work on or near live conductors (Reg 14)*
Working space, access and lighting (Reg 15)*
Competence to prevent danger and injury (Reg 16)*

The regulations convey principles of electrical safety, as applied to any electrical equipment, any work activity having a bearing on electrical safety - in other words they all apply to all electrical systems and equipment, in connection with work activities, whenever manufactured, purchased, installed or taken into use, even if its manufacture or installation pre-dates the regulations.

► FIXED AND PORTABLE ELECTRICAL INSTALLATION

The Electricity at Work Regulations 1989 place a duty on the employer to maintain and conduct tests of all fixed and portable electrical equipment at reasonable intervals. Records of maintenance and electrical testing must be retained for inspection.

 

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8. Cable surrounded by thermal insulation for 400 mm or more has a derating factor of :-
a) 0.63
* b) 0.51 (Table 52.2) p/104
c) 0.55
d) 0.5

9. When automatic disconnection of supply is used as a measure of protection, additional protection by RCD shall be provided for :-
a) Mobile equipment having a rating of greater than 32A
b) Socket outlets in commercial and industrial locations
c) Only for sockets rated at 32A or less where it is reasonable to expect they may be used to supply equipment for use outdoors
* d) (i) Socket outlets with a rated current Not exceeding 20A that are for use by ordinary persons and are intended for general use , and (411.3.3) p/47

10. The maximum disconnection time for a lighting circuit in a commercial premises protected by a ( TT ) system is
0.07
0.4s
5.0s
* 0.2s (Table 41.1) ◄►( 120V < Uo ≤ 230V ) p/46

11. Socket outlets are allowed in a location containing a bath, providing
Located outside of zone 2
Located outside of zone 3
Located outside of zone 2 and protected by a 30 ma RCD
Located 3 metres from the edge of the bath and protected by 30mA RCD (701.512.3)

12. All circuits in a location containing a bath or shower shall have
a) A disconnection time of 0.4s
b) Be installed at a depth of at least 50mm
c) Be installed using earthed conduit
* d) Additional protection by a 30mA RCD (701.411.3.3) p/166

13. When considering if supplementary bonding can be omitted from a location containing a bath or shower, the maximum resistance of extraneous conductive parts connected to the Main Earth Terminal is :-
* a) 1.44[FONT=Times+New+Roman1253]Ω[FONT=Times+New+Roman+Bold1253] 41.3 [/FONT](415.2.2) [/FONT]
b) 0.05[FONT=Times+New+Roman1253]Ω[/FONT]
c) 0.5[FONT=Times+New+Roman1253]Ω[/FONT]
d) 7.6[FONT=Times+New+Roman1253]Ω[/FONT]

[FONT=Times+New+Roman1253]I picked at random ( Amps ) [/FONT]

► R ≤ 50 V/Ia ( 50 ÷ 160 = 0.3 ( I2 / trip sec : ( Type B / MCB . 32A ÷ 160A = 0.2sec ) ◄ R ≤ 50 V/Ia in A.C. ◄◄◄ systems :
► R ≤ 120 V/Ia ( in D.C. ◄◄◄ systems : p/59 !! watch for this

Max loop Impedance : ( Ief )
Zs ≤ Uo / Ia : Zs ≤ 230 ÷ 160A = ( Zs ≤ 1.437Ω )
( Formula : calculating Earth Loop Impedance > ( Zs = Ze + R1 + R2 ) -&-s

435 : Overload Current ( Fault Current Protection / p77
435.1 : Protection afforded by One Device ( RCD/MCB ) p77 ( If ◄ : Short Circuit / Current Protection ) Equal to or Greater than PSCC .

 

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1) 75% of 140 ?
a. 110
b. 115
►c. 105
d. 125

2) Output from a single phase alternator is via..?
►a. Slip Rings
b. Commutator
c. Rotor
d. Field windings

3) Temperature increase on copper has the effect of[FONT=Arial+Bold1252]…[/FONT]?
a. expands length ways
b. expands in thickness
► c. expands in all directions
d. decreases in all directions

4) A poor conductor is[FONT=Arial+Bold1252]…[/FONT]?
a. copper
b. salt water
►c. glass
d. gold

5) If power in is 100watts and power out is 75watts, What is the efficiency ?
a. 25%
► b. 75%
c. 33%
d. 100%

6) One application of the chemical effect is ?
a. magnets
►b. battery
c. lighting
d. Carbon

7) one transmission voltage ?
a. 100kV
b. 200kV
► c. 400kV
d. 500kV

8) Which materials resistance decreases when temperature increases ?
a. copper
► b. carbon
c. aluminium
d. tungsten

9) why do we use the colour yellow for extension leads on-site?
a. Good visibility
► b. denotes voltage
c. it’s a pretty colour
d. denotes length

10) In order to prove a circuit is safely isolated we[FONT=Arial+Bold1252]…[/FONT]..?
a. ask someone if its ok to start work
► b. use approved volt tester to check supply
c. check current flow by switching on
d. look for the red or green light

 

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11) The flow of electrons is basically[FONT=Arial+Bold1252]………[/FONT]..?
a. voltage
►b. current
c. resistance
d. water flow

12) The insulation [FONT=Arial+Bold1252]“[/FONT]Magnesium Oxide[FONT=Arial+Bold1252]” [/FONT]does not work well ?
a. in high ambient temperatures
b. at high operating temperatures
► c. when ingress of moisture has occurred
d. in damp conditions

 

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Remember !

Delta = 3 phase, 3 wire system ◄
Star = 3 phase, 4 wire system ( 3 phase + Neutral ) ◄

Math’s of star and delta systems

The voltages and currents that exist within a three phase system have particular relationships.

Voltages measured across any phase winding are known as ‘Phase Voltages’ and are symbolised VP.

Voltages measured between any pair of lines are known as ‘Line voltages’ and are symbolised VL.

Currents measured through any phase winding are known as ‘Phase Currents’ and are symbolised IP.

Currents flowing along any line are known as ‘Line currents’ and are symbolised IL. Indicate all line and phase voltages and all line and phase currents in the diagrams below.

Delta systems
Voltage Relationships ( VL = VP Therefore ( VP = VL

Current Relationships : IL = 1.732 x IP& IP = IL ►1.732 ◄

Star Systems
Voltage Relationships ( VL = 1.732 x VP & VP = VL ►1.732 ◄

Current Relationships ( IL = IP Therefore IP = IL

You must remember these relationships

[FONT=Comic+Sans+MS0]►Note! Rather than use 1.732 many texts 3 will use the square root of 3 which equates to 1.732. ( √3 ) ◄◄[/FONT]

Progress :
[FONT=Comic+Sans+MS0]►[/FONT]A star connected system has phase voltages of 300v and phase currents of 12A. Calculate line voltages and currents. !! now ask your tutor

[FONT=Comic+Sans+MS0]►[/FONT]If the system above was a delta connected system, calculate the line voltages and currents. !! now ask your tutor

Balanced and unbalanced systems
* Star and delta systems can also be connected as loads. Many three phase motors will be connected in delta since only three wires are required to make the connection

* Star connections are often used as a means of connecting single phase loads to a 3 phase supply, such as in the case of a street of domestic dwellings.

A delta connected load will have identical phase impedances. The impedance of each phase will therefore be identical both in terms of it’s magnitude AND phase angle. When this is the case the current that flows in each phase will be identical and the sum of the currents will be zero. The system is said to be balanced and there is no need for a 4th wire (neutral conductor ).

It is also possible to have a star connected load where the impedance of each phase is identical in terms of it’s magnitude and phase angle. As with the delta system this will lead to identical phase currents and no excess current to flow back to neutral. Again, the system is said to be balanced and there is no need for a 4th wire(neutral conductor).

It is more usual however, to connect dissimilar loads to each phase of a star system. This will result in differences in phase currents which in turn will create some ‘leftover’ current which will flow back along the 4th wire (neutral) to the star point of the supply origin. Such a system is known as an unbalanced system and usually arises as a result of connecting separate single phase loads to each phase of a three phase star connection.

Progress :
What is meant by a 3 phase balanced system ?
Is there any need for a 4th wire (neutral) in such a system ?
How does an unbalanced system differ ?
What function does the 4th wire serve in an unbalanced system.
Which type of 3 phase system is more likely to be found with an unbalanced load ?

 

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Power in three phase systems.
Each phase of a 3 phase system will result in some power consumption.

If the system you are dealing with is balanced there will be an equal power dissipation per phase and therefore it is necessary only to calculate the power dissipated in one phase and multiply the result by 3 (for the 3 phases).

When doing this however you must remember that each phase is an a.c system and as such the power factor is required in addition to phase voltage and current.

Power/phase = VpIp Cos[FONT=Arial+Bold1253]Ø[/FONT]
If the system is balanced :Total Power = 3VpIp Cos[FONT=Arial+Bold1253]Ø[/FONT]

For an unbalanced system it is necessary to calculate each phase power in turn and add them in order to find total power.

An alternative power formula exists which utilises the line voltages and currents:
Total Power = 1.732 x VLIL Cos[FONT=Arial+Bold1253]Ø [/FONT]

These formula apply to both Star and Delta connected systems equally.

Progress :
A star connected balanced load has a phase voltage of 230v and line current of 9A. The system power factor is 0.65. Calculate the total power for the system using both total power formulae. Once you have calculated total power calculate the power per phase.

A delta connected load has a phase voltage of 180v, a phase current of 22A and a phase angle per phase of 25° . Calculate the total power dissipated in the load.

Delta : L1 L2 L3 ,
Star : L1 L2 L3 ( N ◄

Stringbags , I hope this will help your apprentices , something to go on , I knock this up !!
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2330 – level 3 Unit 1 . 301 / paper 3 :- some old papers sorry I forgot to put the Answers up for this , MTF .

 

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Three Phase Electricity Supplies and Systems
The generation and supply of electricity within the U.K is achieved through the use of a 3-phase system.

Voltage – Time !!!!!!

This consists of 3 separate phase conductors along with a neutral conductor transmitting supplies to a given destination

If we took three of these generators (A,B & C) and positioned them equidistantly (120° ) apart) around a rotating magnet we would generate three separate a.c. waveforms each with a phase displacement of ( 120° ) from each other.

* A * / * B * / * C *
Such an arrangement would provide us with a 3 phase supply but we would need 6 conductors in order for it to function. However if we take each coil and interconnect it with the other coils in particular ways we can achieve a three phase supply in either a 3 wire (Delta) system, or a 4 wire (Star) system.

Delta Connection (3-phase, 3 wire system)
Using this method of connection each of the three coil windings are wound in such a way so they each have a start and a finish to their winding.

Then the finish of winding 1 is connected to the start of winding 2. The finish of winding 2 is connected to the start of winding 3. The start of winding 3 is connected to the start of winding 1.

each winding, A,B or C, then mark their starts and finishes.
The supply is then taken from the interconnection of each winding combination as shown above. This means we can achieve a 3-phase system using only 3 wires.

There is an alternative connection that may be made for the provision of a three phase supply which is known as ‘Star’ connection.

Star Connection (3-phase, 4 wire system)
The three coils are also wound so that each has a definite start and finish to their winding.

However to provide a star connection the starts of each winding are connected together at a common point known as the star point. The phase supplies are then taken from the finish of each winding to the point of utilisation. The star point may also have a fourth wire connected to it (known as the neutral conductor) which is then fed along with the three phase conductors to the point of utilisation.

Balanced and unbalanced systems
Star and delta systems can also be connected as loads. Many three phase motors will be connected in delta since only three wires are required to make the connection.

Star connections are often used as a means of connecting single phase loads to a 3 phase supply, such as in the case of a street of domestic dwellings.

A delta connected load will have identical phase impedances. The impedance of each phase will therefore be identical both in terms of it’s magnitude AND phase angle. When this is the case the current that flows in each phase will be identical and the sum of the currents will be zero. The system is said to be balanced and there is no need for a 4th wire (neutral conductor).

It is also possible to have a star connected load where the impedance of each phase is identical in terms of it’s magnitude and phase angle. As with the delta system this will lead to identical phase currents and no excess current to flow back to neutral. Again, the system is said to be balanced ◄◄and there is no need for a 4th wire(neutral conductor).

It is more usual however, to connect dissimilar loads to each phase of a star system. This will result in differences in phase currents which in turn will create some ‘leftover’ current which will flow back along the 4th wire (neutral) to the star point of the supply origin. Such a system is known as an unbalanced system and usually arises as a result of connecting separate single phase loads to each phase of a three phase star connection.

* What is meant by a 3 phase balanced system ?
* Is there any need for a 4th wire (neutral) in such a system ?
* How does an unbalanced system differ ?
* What function does the 4th wire serve in an unbalanced system.

* Which type of 3 phase system is more likely to be found with an unbalanced load ?

 

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[FONT=Arial+Black0]Resistivity [/FONT]
The basis of Ohms Law, of which the Ohmic Triangle we are all so familiar with is derived, is in part derived from these two simple rules…
The resistance of a conductor is Directly Proportional to it[FONT=Arial+Bold1252]’[/FONT]s length ...

i.e. If we double it[FONT=Arial+Bold1252]’[/FONT]s length we will double it[FONT=Arial+Bold1252]’[/FONT]s resistance.
If we half it[FONT=Arial+Bold1252]’[/FONT]s length we will half it[FONT=Arial+Bold1252]’[/FONT]s resistance.
And
The resistance of a conductor is inversely proportional to it[FONT=Arial+Bold1252]’[/FONT]s Cross- Sectional Area[FONT=Arial+Bold1252]…[/FONT].

i.e. If we double the C.S.A we will half the resistance.
If we half the C.S.A we will double the resistance

[FONT=Arial+Bold1252]…[/FONT]at a given temperature.

Note1 - Temperature:
This is an important factor when we consider resistance, as if the Temperature of a conductor increases the resistance increases quite dramatically. All Current Carrying Capacities of cables given in BS7671: 2008 are taken at 30°C. Reasons for an increase in temperature could include Thermal Insulation surrounding a conductor (Ci), Grouping of a number of cables together, all dissipating their heat against each other (Cg), and the Ambient Temperature they are installed in (Ca).

Note2 - Math’s:
Doubling the C.S.A is NOT the same thing as doubling the diameter. Since if the diameter of a cable is doubled it’s C.S.A will increase by a factor of 4 ! Similarly if we half the diameter of the cable it’s C.S.A will fall to a quarter of it’s original value.

Hence if we double the diameter (or radius) of a conductor it’s resistance will fall to one quarter (1/4) of its original value.

If we halve the diameter (or radius) of a conductor it’s resistance will increase by a factor of 4.

 

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Lighting Control (Automatic/Manual :-

The main lighting controls are:-
Switch (Manual)
Contactor (Manual or Automatic)
PIR (Automatic)
Photocell (Automatic)
Timer (Automatic)

Firstly it is important to consider the reason for your lighting & the amount of lighting required. The answers to those two main questions will effect the control equipment used, and is when possibly all aspects of the above could come into play. Remember, larger loads will nearly always require a contactor instead of a switch.

some installation methods are what could be described as ‘acknowledged cable management systems’
These could include :-

Domestic : PVC/PVC Twin
Outdoor Lighting & Power : SWA
Churches : MICC/MIMS
Small Industrial Workshop : Steel Trunking & Metal Conduit drops
Large Industrial Workshop : Bus-Bar Trunking and metal conduit drops
Industrial Kitchens : Galvanised Conduit
Medical Centres : Conduit Drops fixed with Hospital Saddles
Agricultural Buildings : High Impact PVC Conduit
High Explosive Areas : Seamless Conduit
Multi-Story Accommodation : Rising Main (Bus Bar) – or Rising Tray with SWA.

 

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Earth-Fault Loop Impedance :
Earth fault loop impedance is the impedance of the earth fault-current loop starting and ending at the point of earth fault. It comprises the following starting at the point of fault :-

(i) The Circuit Protective Conductor :
(ii) The Consumer’s Earthing Terminal & Earthing Conductor :
(iii) The Earth return Path ( for TT System ) :
(iv) The Path through the Earthed Neutral Point of the Transformer and the Transformer Winding :
(v) The Line Conductor from the Transformer to the Point of Fault :

Path for Earth Fault Current :
Earth Fault Loop Impedance > This is Only a EXAMPLE ‼
The significance of earth fault loop impedance is that a fast disconnection time of the protective device means that a ►high fault current ◄ is required to blow the fuse or to trip the circuit breaker. In order to allow sufficient fault current to flow in order to trip the protective device, the earth fault loop impedance must have a low value. ◄◄

If it exceeds the recommended figure, the circuit must be investigated to find out the underlying reason. The reading obtained should also be compared with the reading of the previous test ► (if this information is available) ◄ to see whether the Zs value is on the increase, which might indicate a potential dangerous condition appearing in the circuit. ► EXAMPLE , if the maximum ( Zs ) was ( 2Ω ) and the test reading was ( 1.2Ω ) on a previous test but is now ( 1.8Ω ) this indicates that the next would produce an unacceptable reading with the possibility that an earth fault occurring in the circuit would not produce enough fault current to operate the overcurrent protective device in either ( 0.4 ) second and ( 5 ) seconds, thus increasing the risk of electric shock to persons using the installation.

* the type of overcurrent protective device and its current rating : and
* whether the circuit feeding socket-outlets or equipment. :

 

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(1) The “ Line “ > Earth Loop Impedance' Tester is plugged into the 13A Socket Outlet :-
Press the Button. The Digital Meter , give a reading directly in Ohms. This is the Line/Earth Loop Impedance.
Repeat procedure (1) for the ( 4 ) number . 13A socket outlets. ► If the Digital Meter give a reading, record the results. ( each One )

If the Digital Meter does not read ‼ ( Houston we have a problem )

By measurement, the Earth Fault Loop Résistance = __________ Ωs

1st 13A S/O : Response of the Impedance Tester ( P-N LED ) ↔ (ON/OFF)↔ ( P-E LED ) Movement of Digital Meter ‼ Reading (Ω s)
2nd 13A S/O ?
3rd 13A S/O ?
4th 13A S/O ?

► Minimum values of the test results to be acceptable. ?
►What is the potential problem if the equipment/cable fails the insulation test ?
► Does all the earth fault loop impedance measured for the functional socket outlets : comply with the requirement as laid down ?

 

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Insulation resistance :
The object of the test is to verify that the quality of the insulation is satisfactory and has not deteriorated or short circuited.

In the event of any test indicating failure to comply, that test and those preceding, the results of which may have been influenced by the fault indicated, should be repeated after the fault has been rectified. ◄◄
Among these tests, items (iv) and (vi)

(iv) Insulation Résistance ,
(vi) Earth Fault Loop Impedance,
Principles of the Inspection & Testing of Electrical Installations.